From the following pairs of ions which one is not an iso-electronic pair ?

The correct option is Fe²⁺, Mn²⁺.

Step-by-Step Explanation:

Two or more chemical species (atoms, molecules, or ions) are said to be isoelectronic if they have the same number of electrons.

To find the pair that is not isoelectronic, we determine the number of electrons in each ion using the formula:
Number of electrons = Atomic Number - Charge on the ion

Let us analyze each option step-by-step:

1. Pair: $O^{2-}$ and $F^{-}$
- Oxygen (atomic number = 8) gains 2 electrons to form $O^{2-}$:
Number of electrons = 8 - (-2) = 10 electrons
- Fluorine (atomic number = 9) gains 1 electron to form $F^{-}$:
Number of electrons = 9 - (-1) = 10 electrons
Since both ions have 10 electrons, they are isoelectronic.

2. Pair: ${\mathrm{Na}}^{+}$ and ${\mathrm{Mg}}^{2+}$
- Sodium (atomic number = 11) loses 1 electron to form ${\mathrm{Na}}^{+}$:
Number of electrons = 11 - 1 = 10 electrons
- Magnesium (atomic number = 12) loses 2 electrons to form ${\mathrm{Mg}}^{2+}$:
Number of electrons = 12 - 2 = 10 electrons
Since both ions have 10 electrons, they are isoelectronic.

3. Pair: ${\mathrm{Mn}}^{2+}$ and ${\mathrm{Fe}}^{3+}$
- Manganese (atomic number = 25) loses 2 electrons to form ${\mathrm{Mn}}^{2+}$:
Number of electrons = 25 - 2 = 23 electrons
- Iron (atomic number = 26) loses 3 electrons to form ${\mathrm{Fe}}^{3+}$:
Number of electrons = 26 - 3 = 23 electrons
Since both ions have 23 electrons, they are isoelectronic.

4. Pair: ${\mathrm{Fe}}^{2+}$ and ${\mathrm{Mn}}^{2+}$
- Iron (atomic number = 26) loses 2 electrons to form ${\mathrm{Fe}}^{2+}$:
Number of electrons = 26 - 2 = 24 electrons
- Manganese (atomic number = 25) loses 2 electrons to form ${\mathrm{Mn}}^{2+}$:
Number of electrons = 25 - 2 = 23 electrons
Since ${\mathrm{Fe}}^{2+}$ has 24 electrons and ${\mathrm{Mn}}^{2+}$ has 23 electrons, they do not have the same number of electrons and are therefore not isoelectronic.